Earthing and Protective Conductors
Earthing and Protective Conductors is the safety spine of the installation. The exam tests whether you can size circuit protective conductors, main earthing and bonding conductors, and prove that the fault path will operate the protective device within the disconnection time.
What earthing must achieve
A line-to-earth fault must be cleared quickly enough to prevent dangerous touch voltages. For TN systems, final circuits not exceeding 32 A must disconnect within 0.4 s at 220 V; distribution circuits within 5 s. The condition is Zs ≤ U0 / Ia, where Ia is the current that trips the device in that time.
Conductor sizing rules
- CPC: line conductor up to 16 mm², CPC the same; line 16–35 mm², CPC 16 mm²; line above 35 mm², CPC at least half the line conductor. Then verify by the adiabatic check S ≥ √(I²t) / k.
- Main earthing conductor: same table applied to the supply line conductor, with a minimum of 16 mm².
- Main protective bonding: not less than half the main earthing conductor, minimum 6 mm², and for TN-C-S not less than 10 mm².
- Bond metallic services at their point of entry, and supplementary-bond exposed and extraneous parts where disconnection times or RCD protection alone are not enough.
Worked example
A 20 A Type B circuit is wired in 30 m of 2.5 mm² single-core copper with a 1.5 mm² CPC, from a board where Ze = 0.35 Ω. Resistance per metre at 20 °C is 7.41 mΩ for 2.5 mm² and 12.1 mΩ for 1.5 mm², so R1 + R2 = 30 × 19.51 / 1000 = 0.585 Ω. Zs = 0.35 + 0.585 = 0.94 Ω. For a 20 A Type B device Ia = 100 A and the limit is 220 / 100 = 2.2 Ω, so the circuit complies with margin. Applying the 1.2 temperature factor to the conductors still leaves it compliant.
Traps that cost marks
Forgetting Ze in the Zs calculation; using 230 V; using a metal conduit or trunking as a CPC without confirming continuity and cross-sectional area; treating a bonding conductor as a CPC; and believing an earth electrode is fault protection in a TN installation.